Which of the following phenomenna lead to decrease in volume of a liquid in an open container?

Analyze the question: A liquid sits in an open container, and its volume goes down. We need the process that removes liquid molecules from the surface into the surrounding air.

Explain the concepts:

• Brownian motion: the random, zigzag movement of tiny particles suspended in a fluid, caused by collisions with the fluid's molecules. It does not remove any liquid.
• Diffusion: the spread of particles from a region of higher concentration to one of lower concentration. It mixes substances but does not by itself reduce the volume of a liquid.
• Evaporation: the change of a liquid to vapour at the surface at any temperature below its boiling point. Molecules with enough kinetic energy escape into the air. In an open container they keep escaping, so the volume of the liquid falls.
• Sublimation: the change of a solid directly to a gas (e.g. iodine, dry ice, ammonium chloride). It involves solids, not liquids.

Solution:

1. The container is open, so escaping molecules are not trapped.
2. The more energetic surface molecules overcome the attractive forces holding them in the liquid and escape as vapour.
3. As this continues, the amount of liquid decreases, so its volume decreases.
4. Only evaporation describes this liquid-to-vapour change. Sublimation involves solids, and Brownian motion and diffusion do not directly cause a volume loss.

Answer: The correct option is C. Evaporation.

Exam tip: Match each process to its state of matter:

• Liquid → gas = evaporation (or boiling)
• Solid → gas = sublimation
• Random motion of suspended particles = Brownian motion
• Spreading of particles = diffusion

Whenever you see "open container" and "liquid decreases," think evaporation.